Demographic Analysis
Fertility:
Concepts and Measures
1
In this lesson, we will cover:
Sources of fertility data
Basic concepts and measurements of fertility,
including age patterns
Direct estimation of fertility from registered births or
from births reported in a census or survey
Overview of indirect methods of fertility estimation
(all related in some way to age structure)
In following lessons, we will explore several of
these methods in greater detail.
Fertility: Overview of Lesson Topics
2
• Fertility is the primary engine of population growth.
Why do we want to measure fertility?
3
Source: U.S. Census Bureau, International Database. http://www.census.gov/population/international/data/idb/region.php?N=%20Results%20&T=12&A=separate&RT=0&Y=1995&R=-1&C=LA
• Knowledge of levels and
trends help us to
formulate or evaluate
policies related to such
population growth.
• Analysis of fertility trends
helps us to predict needs
for maternal health
facilities, teachers, new
schools, etc.
Data sources for fertility estimates
Vital registration
Demographic surveillance system
Censuses
Surveys
Fertility: Data Sources
4
Total births
or births
tabulated
by age of motherQuestions about
1) lifetime fertility2) recent fertility
(usually 12 months)
Recent fertility:
Children born to individual women during a fixed period
(typically 12 months) prior to the interview.
Lifetime fertility (Parity):
Full or partial birth history for individual women (DHS)
Children Ever Born (CEB): A series of questions about
the number of sons and daughters:• born alive and living with the mother• born alive but living elsewhere• born alive but now deceased
Questions on Fertility from Censuses
and Surveys
5
6
Fertility: Estimation Methods
Direct
If data are accurate and
reliable
Can be adjusted for incompletely
reported births
Indirect
If birth data are not accurate or
reliable
If births are heavily
underreported
7
As we progress from one measure to
the next, they will be less influenced by
age structure and hence more
accurately reflect fertility.
• Number of Births
• Crude birth rate (CBR)
• General fertility rate (GFR)
• Age-specific fertility rates (ASFRs)
and
• Total fertility rate (TFR)
Common Measures of Fertility
8
Measures of Fertility: Number of Births
• When would you want to use number of
births?
• When would you not want to?
9
Good for:
1) assessing absolute population growth
2) planning for school enrollments & immunizations
Number of births does not tell us anything about the fertility of the average woman
Large birth numbers may simply reflect a large population of mothers
Measures of Fertility: Number of Births
10
Crude birth rate (CBR) is calculated as the number of births
occurring in a year divided by the population at midyear,
times 1,000. (Question – why midyear?)
CBR = 1000*B / P
For example, the CBR for Hong Kong in 1987 isobtained as follows:
1,000 x 69,811 / 5,613,000 = 12.44(births) (population)
12.4 births per 1,000 population in Hong Kong in 1987.
Measures of Fertility: Crude Birth Rate
11
Which Country Has Higher Fertility?
12
Country A Country B
Births 19,898 15,438
Population (thousands) 315,968 156,207
Crude Birth Rate (per 1000) 12.6 20.4
Source: World Population Prospects 2015. Note: Births (average annual births) and crude birth rates from 2010-2015. Population is the average de facto population between 2012 and 2013.
Yet crude birth rates can be misleading due to the denominator used to
calculate it:
It includes men
It includes women outside childbearing ages
It includes children
Age structure can vary among childbearing women
Implications:
Two populations may have different crude birth rates even if fertility
of women at each age is the same
Women in one society could have a higher birth rate than another
society even though the number of births could be lower at each
age.
Measures of Fertility – Crude Birth Rate
13
General fertility rate: The number of births in a year per
1,000 women ages 15 to 49 years as of the midyear.
GFR for Hong Kong in 1986:
72,221 / 1,469,300 x 1000 = 49.2(births) / (women ages 15 to 49)
There were 49 births per 1,000 women ofreproductive age in Hong Kong in 1986.
Measures of Fertility – General Fertility
Rate (GFR)
14
While improving over the crude birth rate, the general
fertility rate statistic suffers from two limitations:
1. Age structure can vary among childbearing women
2. Although the statistic is clearly defined, it gives us little
intuition about typical number of children per mother
Measures of Fertility – General Fertility
Rate (GFR)
15
An age-specific fertility rate is calculated as the number
of births in a year to mothers of a specific age per
woman (or per 1,000 women) of the same age at
midyear.
Measures of Fertility – Age-Specific
Fertility Rate (ASFR)
16
Age-specific fertility rates are calculated as the
number of births to mothers in a particular age group
in a year per 1,000 women (or per woman) in the same age
group at midyear.
In symbols:
000,1P
Bf
t
xn
t
xnt
xn
Where:
t
xnf is the age-specific fertility rate for
women between ages x and x+n for year t;
t
xnB is the number of births to women between
ages x and x+n in year t; and
t
xnP is the number of women between ages x and
x+n in year t.
Measures of Fertility – Age-Specific
Fertility Rate (ASFR)
17
Measures of Fertility – Age-Specific
Fertility Rate (ASFR)
18
Not affected by differences in the age distribution among women of childbearing ages.
Difficult to use them to make comparisons among populations or within a certain population over time.
Do not easily portray the overall level of fertility.
The total fertility rate (TFR) represents the average
number of children a group of women would have by
the end of their reproductive years if they had
children according to a set of age-specific fertility
rates pertaining to a particular year.
In other words, if a group of women have been
exposed to a given set of ASFR's from age 15 to age
49, the average number of children they would have
by age 50 is the total fertility rate.
Measures of Fertility – Total Fertility Rate
(TFR)
19
The TFR is derived by cumulating the age-specific
fertility rates (per woman) for all ages of women.
When rates are calculated for the seven conventional
5-year age groups, the TFR is the sum of the rates for
each age group, multiplied by five (the width of the
age-group interval).
Measures of Fertility – Total Fertility Rate
(TFR)
20
Deriving Total Fertility Rate (TFR) from
ASFRs
21
Age Specific
Age Births FemalesFertility
(ASFR)
15-19 34,772 860,698 0.0404
20-24 117,697 759,335 0.1550
25-29 85,664 507,188 0.1689
30-34 56,565 422,755 0.1338
35-39 37,072 454,870 0.0815
40-44 14,448 401,344 0.0360
45-49 1,436 319,017 0.0045
0.6201 sum of ASFRs
3.10 Total Fertility Rate
(sum of ASFRs * 5)
ASFR = Births/FemalesAt Each Age
ASFRs show expected fertility at each single year age within each interval. Thus, for a 5-year age interval, multiply sum of ASFRs by 5 to get the TFR, the expected births per woman’s reproductive lifetime.
Summed ASFRs
Table IV-1. Age-Specific Fertility Rates and Total Fertility Rate for
Chile: 1983
Age of Female Number of Fertility
women population births rate
(1) (2) (3) (4) = (3)/(2) x 1,000
15-19 593,262 36,784 62.0
20-24 587,076 81,213 138.3
25-29 505,362 65,236 129.1
30-34 424,186 37,506 88.4
35-39 385,749 17,532 45.4
40-44 325,105 4,929 15.2
45-49 266,575 512 1.9
Sum = 480.4
Sum x 5 / 1,000 = 2.4
The total fertility rate in Chile in 1983 was 2.4 births per woman.
Deriving TFR from ASFRs – Example
from Chile
22
Table IV-1. Age-Specific Fertility Rates and Total Fertility Rate for
Chile: 1983
Age of Female Number of Fertility
women population births rate
(1) (2) (3) (4) = (3)/(2) x 1,000
15-19 593,262 36,784 62.0
20-24 587,076 81,213 138.3
25-29 505,362 65,236 129.1
30-34 424,186 37,506 88.4
35-39 385,749 17,532 45.4
40-44 325,105 4,929 15.2
45-49 266,575 512 1.9
Sum = 480.4
Sum x 5 / 1,000 = 2.4
The total fertility rate in Chile in 1983 was 2.4 births per woman.
Deriving TFR from ASFRs – Example
from Chile
23
Which Country has higher fertility?
24
Country A Country B
Crude Birth Rate (per 1000)
18.2 20.4
Total Fertility Rate (per 1000)
2.25 2.23
Population (thousands) 52,544 155,222
Source: World Population Prospects 2015. Note: Births (average annual births) and crude birth rates from 2010-2015. Population is the average de facto population between 2012 and 2013.
Why would TFR be about the same but
CBR higher in one country?
25
Which Country Has Higher Fertility?
26
Country B Country C
Crude Birth Rate (per 1000)
20.4 16.4
Total Fertility Rate (per 1000)
2.23 2.11
Population (thousands) 156,207 40,944
Source: World Population Prospects 2015. Note: Births (average annual births) and crude birth rates from 2010-2015. Population is the average de facto population between 2012 and 2013.
Age Structure of Women of Childbearing
Age Affects the CBR
27
Age-specific fertility rates (ASFRs) follow a fairly
standard pattern among women in all populations
• start from zero at very young ages
• peak sometime in the twenties,
• declining gradually until again reaching zero around
age 50
Slight variations to the pattern occur, depending on
differences in age at marriage, on the proportion of
women sexually active (mostly within marital unions), or
on the desire and possibility of controlling pregnancies
(mostly by using contraception).
ASFR Patterns
28
0
50
100
150
200
250
15-19 20-24 25-29 30-34 35-39 40-44 45-49
Age-Specific Fertility Rates for 5 Countries
China Haiti India Kenya U.S.
ASFRs Share a Common Pattern
29
0
50
100
150
200
250
300
350
15-19 20-24 25-29 30-34 35-39 40-44 45-49
Age-Specific Fertility Rates for Lebanon
1970 1980 1990 2000 2010
ASFRs Vary Over Time
30
0
50
100
150
200
250
300
350
15-19 20-24 25-29 30-34 35-39 40-44 45-49
Age-Specific Fertility Rates for Mexico
1980 1990 2000 2010
ASFRs Vary Over Time
31
0
50
100
150
200
250
300
350
15-19 20-24 25-29 30-34 35-39 40-44 45-49
Age-Specific Fertility Rates for Uganda
1980 1990 2000 2010
ASFRs Vary Over Time
32
Other fertility measures related to the TFR and helpful
in demographic estimation and analysis include:
The Gross Reproduction Rate (GRR): Constructed
like a TFR, but ASFRs include female births only
Net Reproduction Rate (NRR): The average number
of daughters expected to survive to childbearing
ages – the extent to which a generation of mothers
reproduce themselves. This is like the GRR adjusted
for the impact of mortality.
Other Fertility Measures – the Gross and
Net Reproduction Rates (GRR and NRR)
34
Marital fertility rates can be computed like the measures we
have studied so far. In most cases, the marital version of the
measure includes births to married women in the numerator
and all married women in the denominator.
If not further specified, the Marital Fertility Rate is equivalent to
the GFR for married women.
These measures may be useful in understanding how changes
in marriage affect fertility.
These measures are difficult to interpret in cultures where many
births occur outside marriage, and/or there are different types of
marital status (e.g., consensual unions).
Other Fertility Measures – Marital Fertility
Rates
35
Exercises
In your groups... For your country
Compute the following measures of
fertility:
– CBR
– GFR
– ASFRs
– TFR
Plot the ASFRs. What is the pattern?
36
CALCULATING FERTILITY
ESTIMATES
37
Fertility: Estimation Methods
Direct
If data are accurate and
reliable
Can be adjusted for incompletely
reported births
Indirect
If birth data are not accurate or
reliable
If births are heavily
underreported
38
Indirect Methods
• Use population age-sex structure
– Rele
– Reverse survival
• Use questions on Children Ever Born
– PFRatio
39
Spreadsheets:RELEREVCBR
Spreadsheets:PFRATIOREL-GMPZARFE-2ARFE-3
The importance of fertility data,
techniques for estimating fertility
levels and patterns, and methods for
adjusting fertility data are discussed
in more detail in chapter 4 of the
Census Bureau’s Population
Analysis with Microcomputers.
Fertility: For more information
40